The use of hollow, air-cooled blades and vanes (hereinafter referred to collectively as “airfoils”) is common in modern gas turbine engines. During engine operation, air is flowed into an internal cavity of the airfoils and is discharged through cooling holes present in a section of the airfoil, its platform and/or tip thereof. The passage of air through the airfoil extracts heat from the airfoil surface allowing use of the airfoil even if gas stream temperatures exceed the melting temperature of the alloy from which the airfoil is made. Obstruction of the cooling holes can result in hot spots on the airfoil that may cause cracking or localized melting of the airfoil, and/or degradation of a protective coating that may be present on the airfoil surface.
In the plasma spray coating process, powders may be injected into a high temperature high velocity stream of ionized gases. At the point where the powders are injected into the gas stream, the temperatures can be very high. As a result, the powders are typically molten when they strike the substrate surface.
If coatings are sprayed onto air cooled airfoils, cooling holes can become filled with coating material, requiring a subsequent machine operation to open the holes. This is not only time consuming and therefore expensive, but locating the exact position of each hole is difficult. One known method uses a high pressure stream of gas that is flowed into the internal cavity of the airfoil during the coating operation. The gas is discharged through the cooling holes to deflect the incoming coating particles away from the holes, thereby keeping the holes open. Unless gas is discharged evenly through each of the holes, some holes are still likely to become plugged. A common practice is to drill the holes after coating is applied.
According to a further methodology, plugs are inserted into the cooling holes of hollow, air cooled airfoils prior to the plasma spray coating operation. The plugs are ablative and the plasma spray stream causes a portion of the head of each plug to volatilize during the spray operation. The plugs are sized to provide a sufficient amount of material that can be sacrificed during the spray operation so that at the end of the operation, the plug heads continue to protrude above the surface of the coating. The plugs are removed from the coated component by heating the component at elevated temperatures for a time sufficient to cause complete volatilization to open up the holes.
Sometimes the holes are welded shut before coating is applied, and then are drilled out after the coating process. All such techniques are expensive and time consuming; therefore, improved techniques are desired.